Tunable inductor



Sept. 29, 1970 v. E. DUNN EI'AL TUNABLE INDUCTOR Original Filed Oct. 6,1966 42 3s F/G. 4

INVENTORS VERNON E. DUNN ROY W. ROBERTS JR BIAS MAGNETIC FIELD-H UnitedStates Patent 015cc 3,531,747 TUNABLE INDUCTOR Vernon E. Dunn, PaloAlto, and Roy W. Roberts, Jr., Mountain View, Calif., assignors toMELABS, Palo Alto, Calif., a corporation of California Continuation ofapplication Ser. No. 584,877, Oct. 6, 1966. This application Mar. 7,1969, Ser. No. 806,047 Int. Cl. Hlf 21/08 11.5. Cl. 336-110 2 ClaimsABSTRACT OF THE DISCLOSURE A reciprocal low-loss tunable inductorincluding a coil coupled to a body of ferrite material which is biasedabove ferromagnetic resonance with a variable magnetic field to tune theinductor.

This application is a continuation of application Ser. No. 584,877 filedOct. 6, 1966, now abandoned.

This invention relates generally to a tunable inductor and moreparticularly to a tunable inductor which can be tuned mechanically orelectrically.

Tunable inductors have been known in the prior art. In general, suchinductors depend upon the variation of incremental permeability offerrites or other magnetic material as a biasing magnetic field changesthe ferrite from an unsaturated to a saturated state. The magneticmaterial forms the core for an A-C or signal winding whose inductance isvaried by the incremental variation in the permeability of the core.

One type of tunable inductor has the biasing magnetic field applied inthe same direction as the RF signal or A-C field. In effect then, the RFor A-C field is added directly to the bias field. If the bias field isnot large in comparison to the RF field, it is modulated by the RF fieldproducing such objectionable nonlinear effects as generation of signalsat harmonic frequencies. The loss in the inductor is also increased.

Another type of tunable inductor includes a toroidal core which carriesthe signal winding. The winding covers only portions of the core. Abiasing magnetic field is applied to otherportions of the core to varythe incremental permeability at such portions. Since the entire magneticmaterial is not subjected to the magnetic bias field, there is only asmall change in incremental permeability.

Devices of the prior art have operated with the ferrite in anunsaturated or partially saturated state. It is a well knowncharacteristic of unsaturated ferrites that at certain frequencies,whichdepends upon the material and the biasing field, the lossesincrease rapidly as a result of domain wall motion and ferromagneticresonance.

In general, the prior art tunable inductors have been useful over arelatively narrow range of frequencies, have low power handlingcapacities, have been relatively large and bulky, and have hadrelatively high losses at high frequencies of operation.

It is an object of the present invention to provide a tunable inductorforhigh frequency operation having a wide tuning range.

It'is another object of the invention to provide a tunable inductor for'high frequency operation having low losses.

It is another object of the invention to provide a tunable inductancewhich can handle large amounts of RF power with low loss and with smallor negligible nonlinear effects.

It'is still a further object of the present invention to provide atunable inductor which has small size for a given value of inductanceand unloaded quality factor Q.

It is still a further object of the present invention to provide atunable inductor inwhich RF fields associated with the core are locatedpartially or completely in a fer- 3,531,747 Patented Sept. 29, 1970 ritemedium in such a way that the RF fields are predominantly orthogonal toa biasing field through the same portion of the core, with the operationof the inductor based upon the interaction of the RF fields with theprecessing magnetization of the saturated ferrite.

It is still another object of the present invention to provide a tunableinductor in which the bias field is applied perpendicular to the RFfields and of sufficient strength to saturate the ferrite and bias it sothat the ferromagnetic resonance frequency is greater than the signalfrequency.

The foregoing and other objects of the invention will become moreclearly apparent from the following description when taken inconjunction with the accompanying drawing.

Referring to the drawing:

FIG. 1 is a perspective view of an inductor in accordance with thepresent invention;

FIG. 2 is a perspective view of another inductor in accordance with theinvention;

FIG. 3 is a perspective view of a mutually coupled inductor inaccordance with the present invention;

FIG. 4 is a schematic perspective view of an electrically tunableinductor in accordance with the invention;

FIG. 5 is a schematic view of a mechanically tunable inductor inaccordance with the invention; and

FIG. 6 is a curve showing the permeability as a function of D-C magneticfield.

As previously described, operation of the device of the presentinvention is based on the interaction of the signal or RF field with theprecessing magnetization of saturated ferrites. The permeability in aninfinite ferrite medium, saturated by a D-C magnetic field, of magnitudeH in the Z direction is given by the Polder tensor (Lax and Button,Microwave Ferrites and Ferrimagnetics, Mc Graw-Hill Book Co., New York,1962).

w w w, w 2

ww (T e (3) w ='yH =frequency of ferromagnetic resonance (4) (a 'y4'rrMw angular frequency of RF fields (6) 'y=1.76 107 radians/sec. oersted(7) Where p. is permeability K is orthogonal permeability to isfrequency H is magnetic field 7 is gyromagnetic ratio m=is the magneticmoment per unit volume. I

From this expression, it will be seen that the RF fields perpendicularto the D-C field encounter a permeability which is a function of theapplied D-C biasing field H.

I The permeability holds to very high frequencies and is biasing closeto resonance is limited by the dissipation 1 associated with the ferritein the resonance region. Mathematic ally, the effect of loss is to maket and K become complex. The quality factor Q, can be defined as the aminimum tolerable Q establishes a maximum value for 11..

In principle, it is possible to bias to a low loss region belowresonance to achieve a permeability which varies cludes seriallyconnected windings 36 and 37. A permanent magnet 39 may be employed toapply additional bias fields to the pole pieces 33 and 34 via bridgingmembers 41 and 42 which complete the DC magnetic circuit. As a result,permanent magnet 39 provides a fixed bias field while the coils 36 and37 provide a variable bias field to vary the permeability.

In FIG. '5 there is shown a mechanically tunable inductor. The inductorincludes an inductance member 32. A first fixed steel pole piece 46 isdisposed on one face of a magnet 47 and in contact with inductor 32. Anonmagnetic spacer 48 maintains the tunable inductor and magnetic polepiece in position. A second magnet 49 inwith bias field. In practice,however, with VHF and UHF frequencies and bias fields small-enough tooperate belorw resonance the ferrite remains unsaturated and it maybecome lossy and may not behave in accordance with the theoryestablished above.

Referring particularly to FIG. 6, the variation in permeability withvariation in H is given with ferromagnetic resonance indicated byline-13.'It is seen that the permeability varies with biasing magneticfields considerably below resonance, which is the region invwhich priorart apparatus operated. In the region defined by-the dotted lines 11 and12, as resonance is approached, it is seen that the permability beginsto increase rapidly. However, as pointed out above, the losses may beconsiderable. As the magnetic field H is increased toward resonance, thelosses increase further. Above the resonance condition, to the right ofdotted line 13, the losses arereduced. Itis seen that in this region thepermeabiliy changes appreciably with changes in H.

The tunable inductor of the present invention operates eluding a polepiece 51 is carried by a threaded screw 52 which is 'threadably receivedby an iron pole piece 53. Rotation of the screw 52 will bring the' polepiece 51 towards and away from the ferrite 32 thereby increasing ordecreasing the-reluctance path 54 and varying the D-C magnetic field.The return path for the D-C fields is, of course, through the steel polepiece which threadably receives ihG SCIBWT. Thus, there is providedmechanicaltubing for the inductor.

Tunable inductors of the type described may be used as circuit elementsin various filter applications such as in tunable band pass filters andtunable band rejection filters. They can also be used in tunablematching circuits for matching transmitters to antennas. They canprovide 7 a load impedance which can be tuned to give the best aboveresonance where the ferrite material presents relatively low losses anda wide tuning range.

Referring to FIG. 1, there is shown a simple realization of a tunableinductor in accordance with the invention. The inductor includes aferrite wafer or body 16 disposed within a signal or RF coil 17 so thatthe fields in the coil are run partially through the ferrite material inthe direction of the axis of the coil and substantially parallel to thespaced faces 18 and 19 of the wafer 16. A magnetic bias field H isapplied in the direction of the arrows 21 that are perpendcular to theRF fields. The bias field is of sufficient strength to bias the ferritematerial above resonance.

As a result of the foregoing, the inductance then varies substantiallyas shown by the portion of the curve 22 to the right of dotted line 13,FIG. 6. In this region, the incremental permeability can be controlledby the applied biasing magnetic field 21.

The device shown in FIG. 2 is similar to that in FIG. 1 with ferritewafers 23 and 24 added to sandwich the coil 17 and wafer 16 therebetweento provide a substantially complete path for the RF fields to close uponthemselves. In the embodiment of FIG. 1, there is fixed inductance (airpath) in series with the tunable inductance (ferrite path) and hencethere is a degradation inthe tuning range. The embodiment of FIG. 2provides an improved tunability since the paths for the RF fields arether, however, the winding 27 is coupled to the winding 17 whereby thereis provided mutual tunable inductance between the same to give a tunablemutual inductance.

Referring to FIG. 4, an inductor of the type shown in FIG. 2 andidentified as 32 is shown disposed between the pole pieces 33 and 34 ofan electromagnet which inmatch between a load and its connectedapparatus.

Thus, there has been provided an improved tunable inductor which can beused at high frequencies, which provides tuning over' a relatively broadband of frequencies, and which has low losses.

We claim; I V i 1. 'A tunable inductor including a body of ferritematerial, a coil coupled to said body of material so that the magneticfields associated with the electrical signal carried by the coil are atleast partially within the ferrite material, said body of ferritematerial being disposed within the coil, additional bodies of ferritematerial on each side thereof to sandwich the ferrite body and the coiltherebetween, and means for applying a biasing magnetic field throughsaid ferrite material in a direction such that the signal and biasedmagnetic fields are predominantly orthogonal to one another.

2. A variableinductor including a body of magnetic material, a coilcoupled to said body of material so that the magnetic fields associatedwith the electrical signal carried by the coil are at least partiallywithin the magnetic material, said body of magnetic material beingdisposed within the coil, additional bodies of magnetic materialdisposed adjacent to said body of magnetic material on each side thereofto sandwich the body of material and the coil therebetween, and meansfor applying a' variable biasing magnetic field to said magneticmaterial which biases the magnetic material above ferromagneticresonance throughout the range of variation of the bias field to varythe inductance in response to variations of the biasing magnetic field.

I References Cited UNITED STATES PATENTS

